Fronted solid fuel furnaces with fire-grates have been known thus far: e.g., see U.S. Pat. No. 4,316,445, FR 2 482 702, GB 2 253 050, GB 2 089 969, GB 2 251 302, RU 2 027 953, RU 2 031 315. The presence of grates in those furnaces results in a complicated design and inconvenient maintenance. Besides, obtaining high temperatures in grate-equipped furnaces is prevented by the fact that such temperatures may destroy grates. Fronted charging used in those furnaces leads to excessive air inflow through the furnace door when new portions of fuel are charged. This, in turn, results in a lower temperature in the combustion chamber and, therefore, in an incomplete fuel combustion. Apart from that, a sharp increase of the volume of furnace gases leads to an increasing loss of heat carried away with outgoing gases. The above-listed imperfections bring down the efficiency of fuel combustion and power density of such furnaces and thus prevent them from being extensively used in industry.
There exists another type of solid fuel furnaces, with vertical shaft fuel charging into the combustion chamber. In such furnaces, the ashtray is located under the combustion chamber: see, e.g., EP 0 046248, DE 196 12 403.
To improve the burning efficiency, some types of furnaces provide for air supply to the combustion chamber. The air can be previously heated for an even greater efficiency: see, e.g., GB 1 569 696, DE 32 00 194, DE 32 45 587, EP 0124 945. The air thus supplied is called primary, secondary, or even tertiary air, depending on where and at which stage of burning it is supplied. The air-supply process is thoroughly monitored and controlled to ensure the necessary and safe burning. For this purpose, various auxiliary devices are used, which have to be permanently monitored by operating stuff. The slightest breakdown of these devices can upset the process of burning which, in turn; can result in various undesirable situations, and even in an emergency. Although such furnaces offer more efficiency that those listed above, it is clear that they have a more complicated design and are inconvenient for maintenance. They require special personnel training, and they do not allow for high-efficiency and high power-density burning.